Emulsions appear in a wide range of industries, for example, petrochemical processing, food processing, metal finishing and polishing, textile, paper, cosmetic, pharmaceutical, biotechnology, as well as other industries. It is often necessary to perform separations of one or more components of these emulsions, for example, separation of an aqueous liquid phase (e.g., water) from a non-aqueous liquid phase (e.g., oil) in an emulsion that is composed of either predominately aqueous phase or predominately non-aqueous phase.
For example, in petroleum industries, water is considered a contaminant of the oil products and must be separated from the oil product before further processing, because water may cause considerable corrosion of the processing equipment and may affect the life of the equipment, which may negatively impact the entire plant. Even trace amounts of water in the oil may cause serious problems further down the line. In a contrasting example, oils are a common pollutant in downstream wastewater and byproducts produced in the food and metal industries and should be separated from the wastewater. Separating oil from water (including trace amounts of oil) is a significant challenge. In order to be released back into environment, government regulations require that the oil does not contain more than certain amounts of oil in the water. The maximum allowed quantity of oil for may be 10 ppm of oil or less.
A significant challenge is to reduce the capital costs of energy consumption and reduce or eliminate the use of chemical additives (especially those additives that are considered pollutants and/or additives that otherwise have a negative environmental effect), which are the traditional method of promoting the breakup of emulsions and other mixtures into their components. Another significant challenge is achieving desired levels of separation of oil and water.
There are a number of traditional methods for separating components of emulsions. One of the most common separation techniques is gravity separation. As a primary and low cost treatment step, gravity separation is typically used for separation of emulsions with larger droplet sizes. Gravity separation may be accompanied by a sedimentation process. For example, oil may adhere to the surface of solid particles and be effectively removed by sedimentation. However, gravity separation is not effective for destabilization of emulsions with small droplet sizes, because the time of sedimentation is impractically long (the required time is roughly inversely proportional to the droplet size squared).
In order to separate emulsions with fine droplets, emulsions are typically pretreated chemically to promote coagulation and increase floc size, thereby destabilizing the emulsified phase during gravity separation. In some conventional methods, the emulsion may also be heated to reduce the viscosity, induce stronger density difference, and reduce the surface tension of the stabilizing films between droplets. Other chemical treatment methods increase the acidity or add ionic agents to the emulsion to neutralize the charge of droplets. Chemical treatment methods are energy intensive and may introduce several undesired chemical contaminants. Separation of the additional chemical contaminant may require post-processing unit operations for separation of chemicals, resulting in increased cost and greater risk of environmental pollution.
In addition to gravity separation, other physical methods for destabilizing emulsions include heating, centrifugation, filtration, ultrafiltration (e.g., using membranes), and reverse osmosis. Ultrafiltration (e.g., membrane ultrafiltration) has a smaller chemical footprint than gravity separations and can be somewhat effective for emulsions with small droplet sizes (e.g., smaller than 100 μm). However, the costs associated with ultrafiltration tend to be high (or prohibitive) due to high energy consumption required for ultrafiltration of large volumes, and due to degeneration of the membrane coating materials over time (e.g., such that new membranes need to be provided on a regular basis, further increasing the costs).
Another physical method for separating components of emulsions is electrostatic separation. There are three electrostatic body forces that can be used to induce coalescence. The electric body force in a dielectric liquid, that results from an imposed electric field, can be expressed as:
                              f          →                =                                            ρ              c                        ⁢                          E              →                                -                                    1              2                        ⁢                                          E                →                            2                        ⁢                          ∇              ɛ                                +                                    1              2                        ⁢                          ∇                              [                                                                            E                      →                                        2                                    ⁢                                                            ρ                      (                                                                        ∂                          ɛ                                                                          ∂                          ρ                                                                    )                                        T                                                  ]                                                                        (        1        )            where ρc is volume charge density, ∈ is the fluid permittivity, p is the fluid density, and T is the fluid temperature. The first term on the right hand side of Eq. (1) is the electrophoretic, or Coulombic, force that results from the net free space charges in the fluid. The second term, known as the dielectrophoretic force, arises from the permittivity gradient. The last term, called the electrostrictive force, is important only for compressible fluids.
In these electrostatic separators, it is primarily the second term, dielectrophoretic force, which is exploited to promote the coalescence of droplets in the emulsion. In one conventional technique, two parallel plates are immersed in the emulsion with a small gap spacing between the electrodes. These immersed electrodes are used to induce an external electric field to the bulk of the emulsion. The water droplets in the medium become polarized and positive-negative ends attract each other so that the oil film between two droplets squeezes and is drained. The two adjacent drops may merge together when the layer of the oil between them is ruptured. These droplets do not acquire a net charge. One limitation of this technique is that the polarization force is scaled with the size of droplet. The smaller the droplet size, the larger the field that must be applied. Moreover, the orientation of two adjacent droplets is important. If the angle is not appropriate, two droplets repel rather attract and they cannot be merged—this is a significant limitation of conventional electrostatic separators. The electrohydrodynamic-induced flow and bi-polar attraction (positive-negative attraction) caused by the applied electrophoretic force may induce coalescence of droplets.
The electrohydrodynamic flow generated by interactions of the electric field and fluid flow may also increase the chance of droplet coalescence. AC and DC fields have been used to establish homogeneous or nonhomogeneous fields between the immersed electrodes. Electrostatic separators may be effective in separating droplets as small as a few hundred microns; however, these separators are not effective for smaller droplet sizes in moderate electrical fields.
Although electrostatic separators show some promise, they also suffer from several significant limitations. In conventional electrocoalescencers, both electrodes are immersed in the emulsions. The immediate consequence is that the technique cannot be reliably used when the content of water in the emulsion is high, for example, greater than 40 wt. %. The high content of water may limit the level of applied potential to the electrodes so that even moderate fields may cause electrostatic breakdown. Even when the content of water is moderate or low, the separated water droplets tend to align themselves in the direction of the imposed field and form a chain-like structure across the gap between the electrodes. The formation of this chain may increase the chance of electrostatic discharge and arc across the gap. The electrostatic discharge poses a risk of explosion, as well as corrosion of the electrode or electrode coatings, and increased contamination due to chemical decomposition of oil around the electrodes. Moreover, the electrostatic discharge/breakdown may reduce the rate of coalescence by suppressing the strength of the background electric field, the rate of charging the droplets, and the efficiency of the separator. Additionally, traditional electrostatic separators fail where the aqueous phase has high salt content.
A separation method is needed that is cost-effective, works for emulsions having small droplet size, works irrespective of the salt concentration of the aqueous phase, and does not pose a risk of explosion or require addition of chemical additives to the emulsion.